1. Field of the Invention
This invention pertains to the field of carbon-carbon composites and is most applicable to graphite electrodes. More specifically, the present invention relates to a poorly graphitizing pitch and carbon-carbon composites containing such pitch in the form of a binder, an impregnant, or both, which composites have improved flexural strength and reduced coefficients of thermal expansion.
2. Discussion of Related Art
Carbon-carbon composites are well known and have found commercial use in many different applications in the aerospace, chemical, electrical, metallurgical, nuclear, and other industries.
The popularity of these composites can be attributed to their good mechanical properties as well as their ability to withstand extremely high temperatures and pressures.
A short description of various patents relating to carbon-carbon composites and their applications is set forth in a book entitled "Carbon and Graphite Fibers" edited by Marshall Sitting and published by Noyes Data Corp., New Jersey (1980). In a book entitled "Technology of Carbon and Graphite Fiber Composites" by John Delmonte, published by Van Nostrand & Reinhold Company, New York (1982), a review of the technology of carbon-carbon composites is set forth. Processes for producing such carbon-carbon composites are discussed in the book entitled "Handbook of Composites", Vol. 4, edited by A. Kelly and S. T. Mileiko, published by Elsevier Science Publishers B. V., Holland (1983).
Generally, a carbon-carbon composite comprises a heterogeneous combination of carbon reinforcing material interbonded with a carbon matrix material. The carbonaceous reinforcing material can comprise carbon or graphite fibers, carbon or graphite particles, and combinations thereof. The carbon matrix material can be derived from pitch, organic resin, the thermal pyrolysis of a carbon-bearing vapor, and combinations thereof. As used herein, the term "carbon matrix material" or more simply "matrix material" or "matrix" is understood to mean a material which acts as a binder, an impregnant, or both in the context of a carbon-carbon composite where such matrix material is interbonded with a carbon reinforcing material.
There are two general methods in the prior art for the production of a carbon-carbon composite. One such method entails the chemical vapor deposition of carbon onto a structure defined by carbonaceous reinforcing material. Typically, the carbonaceous reinforcing material comprises carbon felt, woven carbon fibers, or the like, and the matrix is carbon deposited from the thermal pyrolysis of a carbon-bearing gas.
The other general method for producing a carbon-carbon composite comprises fabricating a heterogeneous combination of carbonaceous reinforcing material and a pitch or an organic resin and, thereafter, subjecting this combination to a heat treatment in an inert atmosphere at a temperature of at least about 500.degree. C. to decompose the pitch or organic resin and thereby leave behind a carbonaceous residue bonded to the carbonaceous reinforcing material.
The fabrication of the heterogeneous combination is accomplished by adding the pitch or organic resin to the reinforcing material at a temperature at which the pitch or resin is in a liquid state so that it can wet the carbonaceous reinforcing material and infiltrate into and throughout this material and act as a binder therefor.
The heat treatment to decompose the pitch or organic resin is usually referred to in the art as "carbonization". The carbonaceous residue arising from the decomposed pitch or resin is sometimes referred to in the art as a "char". When pitch is used, the carbonaceous residue is generally referred to as a "coke". In order to interbond the matrix material with the reinforcing material, the carbon-carbon composite structure must at least be subjected to a carbonization heat treatment.
Depending upon the particular end use of the carbon-carbon composite that is being prepared, the carbonized composite may then be subjected to yet an additional heat treatment step. In this further heat treatment step, usually referred to in the art as "graphitization", the composite is heated to a temperature of at least 2600.degree. C. to cause the carbon atoms in the filler and in the binder to orient into a graphite lattice configuration. This ordering process produces graphite with its intermetallic properties that make it useful for many applications.
In the production of a graphite electrode, which is a specific form of carbon-carbon composite, coke filler particles are mixed with a pitch binder which pitch is at a temperature such that it is in its liquid state thereby uniformly dispersing the particulate filler and allowing the desired article to be formed in a subsequent extrusion or molding step. After being formed, the "green electrode", as it is commonly known in the art, is then subjected to a first heat treatment step which is known as "baking" in the electrode art but which is substantially similar to the "carbonization" step discussed earlier. In this step, the thermoplastic pitch binder is converted to solid coke. The graphitized electrode is then formed by subjecting the baked electrode to a temperature of about 2600.degree. C. to 3000.degree. C. for a period of about 0.5 to 20 hours.
In the production of carbon-carbon composites, particularly graphite electrodes, one of the fundamental objectives is to obtain a composite having a high density and correspondingly low porosity. Relatively high porosity in a composite leads to an undesirable concomitant loss in both strength and other mechanical properties.
Porosity in a carbon-carbon composite generally arises as a result of volatilization of the low molecular weight components of either the pitch or the organic resin primarily during carbonization or baking and to some extent during graphitization. The coke yield of a pitch binder is generally about 40 to about 60 percent by weight after a heat treatment to about 500.degree. C. at atmospheric pressure. This yield declines slightly as the temperature is increased to about 1000.degree. to 1400.degree. C. Similar carbon yields (char yields) are evident with organic resin binders. This loss of about 40 to 60 percent of the original binder as a result of its decomposition during carbonization of the composite structure results in voids being created producing a structure having a low density and reduced strength.
Various expedients have been employed in the prior art to avoid the formation of porosity after heat treating a combination of carbonaceous reinforcing material and a pitch or organic resin binder. One method, as discussed in the aforementioned "Handbook of Composites", is to apply pressure to the combination of the carbonaceous reinforcing material and the pitch binder throughout the carbonizing heat treatment in order to increase the carbon yield.
Another method, as also discussed in the "Handbook of Composites", is to impregnate the carbonized composite structure with the pitch or organic resin followed by an additional heat treatment, usually under pressure, to carbonize the impregnant and thereby attempt to fill any voids that were created by the initial carbonization step. Generally, at least two impregnations are used with each impregnation followed by a heat treatment under pressure.
These techniques, however, are not completely effective since voids caused by volatilization in the binder and/or impregnant are still created during the heat treatment steps.
In an effort to remedy this problem, the art has resorted to retaining the volatile components of the pitch so as to improve both the pitch yield as well as the carbon yield by polymerizing these volatile components with chemical reagents thereby forming higher molecular weight constituents which are not readily susceptible to volatilization at carbonization temperatures.
In U.S. Pat. No. 4,096,056, for example, tar precursors are heated while blowing an oxygen containing gas into the reactor to polymerize the volatile components and thereby produce higher yields. In German Patent No. 1,015,377, nitro-functional group-containing aromatic compounds are added to pitch in order to achieve the same results by polymerization with these nitro-functional groups. British Patent Application No. 2,045,798 also shows a process for preparing a pitch from a tar which process comprises mixing the tar with a nitrating agent.
In the "Chemistry of Carbonization" and the references disclosed therein set forth in Carbon, Vol. 20, No. 6, pp. 519-529 (1982), it is disclosed that sulfur has also been used extensively as an additive to increase carbon yield.
Generally, however, as the carbon yield increases, the rheological characteristics of the pitch also suffer. Thus, the pitch must exhibit a viscosity which allows for its being appropriately used as a binder and/or impregnant at specific operating temperatures of various commercial processes. For example, in the manufacture of graphite electrodes, coal tar pitch binder is mixed with petroleum coke filler at about 150.degree. to 170.degree. C. and extruded into a green electrode at 100.degree. to 130.degree. C. Usually, the Mettler softening point is used as the rheological criterion for pitches. As used herein, the term, "softening point" refers to the temperature at which the viscosity of the pitch is reduced to the degree required by the Mettler softening point method of ASTM D 3104-75. Typical electrode pitch binders have Mettler softening points of about 100.degree. to 120.degree. C. Further polymerization can result in a pitch having a softening point of as much as 250.degree. C. or more.
In addition to increasing the softening point, polymerization of precursor tars or pitches with a chemical reagent may also affect the graphitizability of the resulting polymerized pitch or resin. In an article entitled "Chemical Changes During the Mild Air Oxidation of Pitch" by J. B. Barr and I. C. Lewis, set forth in Carbon, Vol. 16, pp. 439-444 (1978), it is taught that oxidation reactions may modify the structure of the pitch by developing a network of cross-links between the molecules, thereby leading to the formation of disordered carbon structures which may prevent graphitization. In the aforementioned "Chemistry of Carbonization", it is taught that high sulfur addition may also result in the carbon being non-graphitizing. In U.S. Pat. No. 4,066,737, which is directed to a method for making isotropic carbon fibers, it is disclosed that carbonization of highly cross-linked macromolecular structures containing oxygen, nitrogen or sulfur, form rigidly cross-linked aromatic planes which prevent further conversion to a graphite structure. Finally, in the Journal of Materials Science, Vol. 18, pp. 3161-3176 (1983) in an article entitled "Review--Science and Technology of Graphite Manufacture" by S. Ragan and H. Marsh, it is disclosed that while the addition of sulfur or nitro-aromatic compounds may increase the carbon yield of a pitch binder upon baking, such addition, however, may adversely affect the graphitizability of the binder.
The need to have good graphitizability of a binder in the production of a carbon-carbon composite, particularly a graphite electrode, has long been well accepted. Good graphitizability has been correlated with desirable electrical and thermal properties in the resulting composite structure, particularly graphite electrodes.
In the Ragan and Marsh article, it is taught that binders used in the manufacture of electrode and graphite products need to fulfill various specifications. One such specification is the ability of the binder to produce a graphitized binder coke so as to improve electrical and thermal properties. The need to produce a binder coke that can be graphitized is once again taught in a section entitled "Carbon and Artificial Graphite" of the Kirk-Othmer: Encyclopedia of Chemical Technology, Vol. 4, Third Edition, pp. 156-631 (1978).
The need for graphitization also carries over to the reinforcing material. In an article entitled "Here is What's New in Delayed Coking" appearing in the Journal of Oil and Gas, 68:92-6 (1970) by A. Kutler, et al., it is taught that an easily graphitized coking material is desirable for it produces the qualities of low porosity, good conductivity and low coefficient of thermal expansion. This teaching is again repeated in a book entitled Recent Carbon Technology edited by T. Ishikawa and T. Nagaoki (I. C. Lewis as the English editor), JEC Press Inc. (1983), pp. 31-34.
A low coefficient of thermal expansion is extremely important in carbon-carbon composites, especially graphite electrodes. In addition to being directly indicative of the amount of thermal expansion exhibited by the carbon-carbon composite, the coefficient of thermal expansion is also indicative of many other properties as well. For example, both graphite and carbon electrodes undergo extreme thermal shock in their use in open-arc furnaces and submerged-arc furnaces, respectively. An electrode having a low coefficient of thermal expansion has a high resistance to such thermal shock. Moreover, a low coefficient of thermal expansion is also indicative of less breakage, less consumption, as well as low electrical resistance. In carbon-carbon composites other than electrodes, a low coefficient of thermal expansion may be necessary for applications in which a close tolerance in the overall dimensions of the composite structure is required.